In the context of wind turbine rotor blades it is generally intended to reduce the loads on the wind turbine blades. Until now the wind turbine blades and supporting structures are usually dimensioned so that they are able to withstand the dynamic loading that can occur under the conditions to which the turbine is certified. But for large wind turbines very heavy designs with enormous operational loads are the result of this practice. Consequently, various methods have been developed for an active reduction of operational loads.
One method for an active reduction of operational loads is cyclic blade pitch as it is, e.g. described in U.S. Pat. No. 4,298,313. In the cyclic blade pitch the blade's pitch angle is varied during a rotational period of the rotor. Cyclic blade pitch can be regulated by simple or advanced means, using the normal blade pitching system. Pitch control systems are, e.g., described in EP 1 666 723 A1 or US 2006/0145483 A1. A disadvantage of cyclic pitch is wearing on pitch actuator systems and pitch bearings. Moreover, the regulation speed is limited, typically with response times in the order of seconds, because cyclic pitch requires the movement of large inertial masses in bearings with large frictional moments.
More recently various forms of active flap regulation have been described. In an active flap regulation the pitch setting is typically not changed. Instead, the aerodynamic characteristics of the blade are modified by a flap action, thereby enabling a desired response to the structural loading. A method of controlling aerodynamic load of a wind turbine by means of flaps is, e.g. described in US 2006/0145483 A1. A specific advantage of active flap regulation is a faster regulation speed than for cyclic pitch. Due to the limited inertia of a flap the regulation reaches typical response times in the order of tenth of seconds. The main weaknesses of known flaps are the required complex actuators, typically electrical systems that may be highly vulnerable to especially lightning damage.